Assessing the effect of dissolution on key Pliocene Mg/Ca-based tropical temperature records

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

The tropical Pacific harbors the largest mass of very warm water of the world's ocean and is a primary source of heat and moisture to the atmosphere. Changes in the temperature gradients across the tropical Pacific drive major changes in tropical circulation and global climate. Studies of the tropical Pacific during the Pliocene Epoch (three to five million years before present) can provide important insight into how the Earth's climate behaved at a time in the geologic past when atmospheric carbon dioxide concentrations were similar to today (~100 ppm higher than preindustrial values). Reconstructions of Pliocene tropical Pacific temperatures using geochemical techniques agree with some Pliocene climate model simulations, but not others. This unresolved issue is the focus of this research project. The aim of this project is to re-evaluate the Pliocene temperature estimates that are based on the magnesium to calcium ratio (Mg/Ca) of fossil shells by quantifying whether changes in the preservation of the shells resulted in biased temperature estimates. The results of this work willl provide important validation of climate models used to simulate warm climate states. This project involves the professional training of a graduate student, training of undergraduate students from underrepresented groups in STEM fields, and outreach to undergraduates in the San Francisco and Monterey Bay Areas. Many of the existing sea surface temperature records from the tropical Pacific are derived from the Mg/Ca composition of planktonic foraminifera, which is susceptible to dissolution on the seafloor and in the sediments; enhanced dissolution results in a cold temperature bias. It is standard practice to correct for the effect of dissolution using carbonate ion concentrations at the location and water depth of the sediment core. However, dissolution can change with time and the effects of changing dissolution on Mg/Ca-based temperature estimates is the most likely source of proxy bias in the tropical Pacific. This project will measure the boron to calcium (B/Ca) ratio of benthic foraminifera in the same samples that were used to produce published Mg/Ca-temperature estimates in the tropical Pacific over the last 5 Myrs. B/Ca is a proxy for carbonate ion concentration, and thus can be used to properly adjust the Mg/Ca-temperature estimates to account for dissolution. The new B/Ca analyses will be applied to evaluating potential dissolution biases at Ocean Drilling Project (ODP) Hole 806 in the West Pacific warm pool, but will also, using a more limited set of analyses, evaluate ODP 847 in the East Pacific cold tongue. Both the absolute temperatures and the West-East temperature difference will be quantified once the effect of dissolution is taken into account, and compared to climate model simulations of the Pliocene warm period.